This invention relates to a method of preparing calcined ceramic powders, in particular a method using microwave radiation as the energy source.
Powdered ceramics have a large number of applications. Varistor powder is added to polymers to make stress-grading materials for high voltage products. See, e.g., WO 97/26693 (1997); FR 2,547,451 (1984) and EP 0,005,387 (1979). Powdered lithium manganese oxide (LiMn.sub.2 O.sub.4) is used in rechargeable batteries. Powdered phosphors are used in fluorescent lights. Powdered clays find various uses, such as fillers for polymers and thickeners for pastes.
Ceramics are generally prepared by heating (firing) a green body made of a precursor material at a high temperature, between 600.degree. and 1,400.degree. C. One method of obtaining a powdered ceramic is to prepare the ceramic in bulk by firing and then crushing it. This approach is disadvantageous because an extra step is introduced and because crushing may destroy or damage important microstructural characteristics of the ceramic. For instance, the non-linear resistivity of varistor material is attributed to grain boundary effects. Crushing has been found to damage these boundaries, so that the crushed powder does not have the same non-linear resistivity as the bulk material. Bulk heating is energy-inefficient, as heating through a thick body is required.
An alternative is to calcine a precursor powder in a kiln. However, the precursor material may coalesce or fuse together--i.e., sinter--if the process is not performed properly. Processes in which a flux-containing precursor powder is fired, as occurs in the preparation of varistors and phosphors, are especially prone to fusing.
Thus, it is desirable to develop another method of preparing calcined ceramic powders which is more efficient, in terms of energy consumption, capital equipment requirements, and the number of steps required.